The proposed research is concerned with an examination of the active and passive components of transport in acidification by the isolated urinary bladder of the water turtle. The role of the different components and the factors controlling them will be defined. Current studies indicate that the rate of active H ion transport is controlled by the H ion gradient across the luminal membrane, by the quantities of CO2 that can be hydroxylated within the epithelium and by as yet poorly defined energetic factors. It is the objective of the research to clarify the coupling between H ion transport and metabolism under conditions of maximal H ion secretion and under conditions of secretion against an electrochemical gradient where the proton-motive force of the pump can be evaluated. The humoral regulation of the transport rate and the proton-motive force will be explored by examining the effects of aldosterone, parathyroid hormone and other hormones. In the presence of ambient bicarbonate, the net rate of acid secretion is modified by the operation of an anion exchange transport system which secretes HCO 3 negative ion into the urinary compartment in exchange for Cl negative ion absorbed. The dependence of this transport system on metabolic energy and the factors controlling its transport rate will be studied. The characteristics of this transport system will be compared with those of the H ion pump to arrive at a model for the overall regulation of acidification.